The present invention relates to stable aqueous polymer dispersions and a process for their preparation. More particularly, aqueous polymer dispersions are prepared by forming a polymer salt in a hydrophilic organic solvent and removing the hydrophilic organic solvent from a solvent blend of water and the hydrophilic organic solvent at temperatures which minimize the mean particle size of the polymer in the resulting aqueous dispersion of the polymer and/or removing the organic solvent from the polymer/organic solvent/water blend such that inversions do not occur.
Protective and decorative coatings based on polyester, alkyd and acrylic polymers have been increasingly used in many applications because they provide a wide range of strength, flexibility, toughness, adhesion, degradation resistance and other film properties. Many are solution polymers because they are prepared and applied as solutions of organic polymers in organic solvents.
Environmental concern over the use of organic solvents has become increasingly important to the coating industry. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to volatile organic compounds (VOCs) which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and coating industry which uses organic solvents that are emitted into the atmosphere.
The use of aqueous dispersions of polymeric vehicles, or alternatively, high solids polymeric vehicles are two general approaches that have been used to reduce VOCs in coating compositions. Previous approaches for conventional aqueous systems have limited the molecular weights of the polymers used in the polymeric vehicle, which limits the impact resistance and other properties of the coating binders and films resulting from the polymeric vehicles.
The conventional process for the preparation of aqueous polymer dispersions is emulsion polymerization, in which hydrophilic assistants (emulsifiers and/or protective colloids) are used in order to ensure the stability of the dispersion. These assistants make the films produced from the dispersions somewhat sensitive to water.
Another way to reduce VOCs is to use water as a medium for the film-forming components in the polymeric vehicle. One approach is to make a polymer such as a polyester, alkyd, acrylic or epoxy polymer having carboxyl groups, or other ionizable groups and acid number in amounts effective such that when the carboxyl groups are converted into salts with a neutralizer such as an amine, they will permit the polymer or oligomer to be dispersed in a mixed solvent system which includes an organic solvent and water. The salt, such as an amine salt, of the oligomer or polymer disperses into the mixed water/organic solvent system with the formulation of a dispersion as opposed to a solution. This is commonly known as a water reducible system.
Organic solvent has typically been removed from water reducible systems through two routes. The first route utilizes low temperature boiling solvents that are water miscible and often form low temperature boiling azeotropes with water. These types of processes result in the loss of neutralizer and require addition of make-up neutralizer in the process. Further, use of insufficient solvent in these systems results in the polymer salt being insoluble with addition of water. Hence, an inversion takes place as solvent is stripped from the system.
The second route typically utilizes organic solvents that boil at temperatures greater than 100xc2x0 C. and also often form azeotropes with water. Distillation proceeds at higher temperatures which, if the salt is an amine, will result in loss of amine, cause undesirable particle size variation in the dispersion and undesirably increase the average particle size of the resin in the dispersion. If distillation continues too long and too much amine is stripped, the dispersion will become unstable and fall apart.
To keep viscosities low, in some water reducible systems molecular weights of polymers have been kept low. As a result, these systems have required large amounts of cross-linker and cross-linking to achieve coatings performance. The use of higher molecular weight polymers in water reducible systems often results in unacceptably high processing viscosities due to the inversion from water in oil to oil in water. In addition, removal of water and solvent results in loss of neutralizer as described above, destabilization of the dispersion and undesirable large resin particle size.
Two component waterborne polyurethane coating systems based on polymeric dispersions and emulsified polyisocyanates, have been developed. However, out-gassing is generally a major problem during application of these systems. Isocyanate can react with water and produce CO2, resulting in gassing when the film is cured.
Another application problem for these 2 component waterborne polyurethane coating systems is that a rapid drop in viscosity usually occurs when the part B component, polyisocyanate resin, is added to the part A component of the paint. This makes it difficult to control the rheology of the final paint, resulting in poor sag resistance and film buildup. Because of this, the application is usually limited to horizontal surfaces. Therefore, there remains a need to develop a dispersion for a 2-component waterborne polyurethane application without gassing and viscosity drop problems.
Examples of processes for forming acrylic polymer dispersion are described in U.S. Patent Nos. 5,319,019, 5,356,988 and 5,380,771. In each of these processes water immiscible solvents are used and solvent is stripped at high temperatures. High temperature stripping results in inversions which increase viscosity and result in increased particle size.
An object of the invention is to provide a polymeric vehicle which will reduce or minimize VOCs.
Another object of this invention is to provide polymeric vehicles which are not only low in VOCs, but which provide coating binders with good film properties such as hardness and impact resistance.
Another object of this invention is to provide a process for preparing stable aqueous polymer dispersions having higher solids content with resins having low acid values and higher molecular weights, yet also retain low processing viscosities while preparing the dispersions.
Yet another object of this invention is to control the viscosity to low levels of a liquid polymeric vehicle or liquid formulated coating composition with the use of water and with the minimization of organic solvents for such control.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.
The present invention is directed to a dispersion process effective for providing a stable water dispersion of high molecular weight polymers. The aqueous dispersions of the invention have less than about 5 weight percent organic solvent, at least about 30 weight percent solids, and a viscosity of less than about 20.0 poise at about ambient temperature. In an important aspect of the invention, processing temperatures are minimized, mean particle size of the resins in the dispersions are kept to size of not more than about 400 nm and the dispersion process is inversionless.
In one aspect, the present invention is directed to a 2 component system. The first component includes an aqueous dispersion of a neutralized polymer in water and a co-solvent. The second component includes an isocyanate cross-linker containing an isocyanate compound. The first and second component when mixed are effective for providing a coating binder film. In this aspect of the invention, the polymer is a condensation polymer, addition polymer or hybrid of condensation and addition polymers. In this aspect of the invention the polymer has an acid value of from about 4 to about 70 prior to neutralization, and a solubility of at least about 50 weight percent in a hydrophilic organic solvent. Furthermore, the solvent typically has a solubility of at least 5 weight percent in water. The aqueous dispersion has less than about 2 weight percent organic solvent, at least about 30 weight percent solids, a viscosity of less than about 20 poise at a temperature of about 25xc2x0 C., and a mean particle size of not more than about 300 nm.
In accordance with the dispersion process of the invention, an ionic functional polymer is synthesized in a hydrophilic organic solvent, or synthesized neat and subsequently mixed with organic hydrophilic solvent. In an important aspect of the invention, the polymer which is formed has an acid value of at least about 4, and has a solubility of at least about 50 weight percent in the hydrophilic solvent, based on the weight of the polymer and solvent, and the hydrophilic solvent has a solubility in water of at least about 5 weight percent at processing temperatures, based on the weight of the solvent and water mixture.
After the polymer is fully dissolved in the organic solvent, an amount of neutralizer is added effective for providing a dispersion of the polymer upon addition of water and removal of solvent. The neutralizer may be any salt-forming base compatible with the ionizable functional polymer such as sodium hydroxide or an amine. In certain aspects the neutralizer is an amine type which is selected from the group consisting of ammonia, triethanol amine, dimethyl ethanol amine, and 2-amino-2-methyl-1-propanol. Not all of the ionizable groups on the polymers need to be reacted with the base (or neutralized).
Depending on the type of ionizable groups present in the polymer it may be important that the polymer is neutralized before it is blended with water so that water dispersible neutralized ionizable groups are generally evenly distributed throughout the polymer. After the formation of the polymer salt solution in the hydrophilic organic solvent, and mixing that solution with water to form a water/organic solvent/polymer salt blend, the organic solvent and water are removed or stripped from the blend at a duration, temperature and pressure effective for providing an aqueous dispersion having a resin mean particle size of not greater than about 400 nm, a polymer concentration of at least about 30 weight percent and an organic solvent concentration of less than 5 weight percent. In an important aspect, where the neutralizer is an amine or ammonia, the mean particle size of the resin is maintained with a stripping temperature of not more than about 65xc2x0 C. at a pressure which permits such a stripping temperature.
In the final step of the process of the invention where the hydrophilic solvent is removed from the water/organic solvent/polymer salt blend, there is a steady small increase in viscosity without inversions due to the increase in non-volatile materials (NVM). Nevertheless, no viscosity spike is observed during processing. Inversions should be avoided because they will cause high processing viscosities. High processing viscosities will cause use of energy or heat (such as from applying high shear to maintain mixing), or in the alternative, will cause the use of large relative amounts of water or organic solvent which will cause disposal problems which increase processing costs and also increase raw material costs because the processing aids are ultimately disposed of.
It also should be recognized as important that the organic solvent is removed such that inversions do not take place during the strip of the organic solvent. With the elimination of inversions during processing, the viscosity of the system remains in the range of about 0.1 poise to about 20 poise at a temperature of about 25xc2x0 C., and preferably about 1 to 20 poise, throughout the process, and a high viscosity spike normally attributed to inversion processes does not occur. Because a high viscosity spike is never encountered during processing of the dispersion, a higher solids contents can be achieved. In a very important aspect of the invention, at 25xc2x0 C. the dispersion will generally have a viscosity of less than about 10 poise.
In the alternative, the temperature of the strip is below the temperature at which substantial loss of neutralizer would otherwise occur. The mean particle size of the resin does not exceed more than about 400 nm, with a typical mean particle size range of about 40 nm to about 200 nm.
In an important aspect of the invention, polymers which can be dispersed in accordance with the present invention include condensation polymers, addition polymers, and polymers which are hybrids between condensation and addition polymers.
In an important aspect of the invention where the dispersion includes a condensation polymer the solids levels may be at least about 40 weight percent. In the aspect of the invention directed to condensation polymers, the resins have a number average molecular weight (Mn) of about 2,000 to about 10,000 and an acid value of at least about 4.
In another aspect, the polymers are addition polymers, such as acrylic polymers. In an important aspect, where the dispersions include an addition polymer, the solids level of the dispersion may be at least 30 weight percent. In the aspect of the invention directed to addition polymers, such as acrylic polymers, the acrylic has a Mn of greater than about 3,000 and an acid value of at least about 4.
In another aspect of the invention, the polymer being dispersed in accordance with the invention is a hybrid between a condensation polymer and an addition polymer. In an important aspect, when the dispersion includes a hybrid polymer, the solids level of the dispersion may be at least about 30 weight percent. In the aspect of the invention directed to hybrid polymers, the hybrid polymers have a Mn of at least about 2,000 and an acid value of at least about 4.